Isocyanates are widely used as production raw materials of such products as polyurethane foam, paints and adhesives. The main industrial production method of isocyanates comprises reacting an amine with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a method for producing isocyanate compounds is sought that does not use phosgene.
Although examples of such methods include a method for synthesizing aliphatic isocyanate from an aliphatic nitro compound and carbon monoxide, and a method for converting an aliphatic amide compound to isocyanate by Hoffmann decomposition, both of these methods have poor yield and are inadequate for industrial application.
Methods for obtaining an isocyanate and a hydroxy compound by thermal decomposition of N-substituted carbamic acid ester have long been known, an example of which may include the method of A. W. Hoffmann (see Non-Patent Document 1). This method enables a high yield to be achieved more easily than the methods described above, and the basic reactions employed in this method are indicated below:R(NHCOOR′)n→R(NCO)n+nR′OH  (A)(R′NHCOO)nR→nR′NCO+R(OH)n  (B)(wherein R represents an organic residue having a valence of n, R′ represents a monovalent organic residue, and n represents an integer of 1 or more). Thermal decomposition represented by the above general formulas is reversible, and although the equilibrium thereof is biased towards the N-substituted carbamic acid ester on the left side at low temperatures, the side with the isocyanate and hydroxy compound is advantageous at high temperatures.
In this manner, thermal decomposition of N-substituted carbamic acid ester is associated with harsh reaction conditions, such as being carried out at high temperatures, as well as the concomitant occurrence of various irreversible side reactions.
As indicated in the publication by Schiff (see Non-Patent Document 2) and the research by E. Dyer and G. C. Wright (see Non-Patent Document 3), examples of such side reactions may include those resulting in the formation of substituted ureas, biurets, urethodiones, carbodiimides and isocyanurates.
These side reactions not only cause decreases in selectivity and yield of the target isocyanate, but also induce the formation of polymers during the production of polyisocyanate in particular, and depending on the case, can cause a situation that makes long-term operation difficult, such as causing the reactor to be clogged by precipitation of polymeric solids.
The majority of undesirable side reactions occur at higher temperatures, have a long reaction time, and the formed isocyanate tends to increase the longer the duration of contact with each component of the reaction mixture.
Various methods have been proposed thus far relating to the obtaining of a favorable isocyanate yield by inhibiting the formation of products of undesirable side reactions during thermal decomposition of N-substituted carbamic acid esters.
First, with respect to methods for producing an intermediate in the form of N-substituted carbamic acid ester, several methods have been disclosed for producing N-substituted carbamic acid ester that do not use phosgene. For example, Patent Document 1 describes a method for oxidative urethanation from a primary amine, carbon monoxide and an aliphatic alcohol or aromatic hydroxy compound using a precious metal catalyst. However, since this method uses highly toxic carbon monoxide and an expensive precious metal catalyst, it has problems such as requiring a complicated procedure and excessive cost to recover the catalyst from the product in the form of N-substituted carbamic acid ester.
In addition, Patent Document 2 describes a method for producing N-substituted carbamic acid-O-aryl ester by reacting an N-alkyl-N,N′-dialkyl urea, an aromatic hydroxy compound and hydrogen chloride gas. However, this method uses corrosive hydrogen chloride gas, consumes an expensive and uncommon urea compound, and has the problem of requiring a complicated procedure and excessive cost to recover the N-substituted carbamic acid-O-aryl ester from a hydrochloride of N,N′-dialkylamine formed as a by-product.
Methods using urea or a carbonic acid derivative (such as carbonic acid ester or carbamic acid ester) have been proposed as methods for producing N-substituted carbamic acid ester that are alternatives to methods using expensive raw materials or catalysts and the like in the manner of the methods described above.
Patent Document 3 describes a method for producing aliphatic N-substituted carbamic acid ester that does not use phosgene in which a 1,3-di-substituted urea is produced from a primary amine and urea in a first stage, and an N-substituted carbamic acid ester is produced by reacting the 1,3-di-substituted urea with a hydroxy compound followed by separating and recovering the primary amine produced as a by-product and returning it to the first stage in a second stage. However, not only is the yield of the N-substituted carbamic acid ester formed low, but recycling equipment is required for the primary amine, thereby making the process extremely complicated and making this method unsatisfactory for industrial application.
An example of a method for producing N-substituted carbamic acid-O-alkyl ester using urea or a carbonic acid derivative is disclosed in Patent Document 4 in which a diamine, an alcohol and urea are reacted to convert to an N-substituted carbamic acid-O-alkyl ester. Patent Document 5 discloses a method for producing N-substituted carbamic acid-O-alkyl ester after first producing bis-urea from an aliphatic primary polyamine, urea and alcohol, while Patent Document 6 discloses a method for producing N-substituted carbamic acid-O-alkyl ester by partially reacting urea and alcohol in a first step and then supplying a diamine in a subsequent second step.
However, since the N-substituted carbamic acid-O-alkyl esters produced by these methods are thermally extremely stable, a thermal decomposition reaction that produces isocyanates from these N-substituted carbamic acid-O-alkyl esters requires a high temperature that causes the formation of polymers due to undesirable side reactions (for example those represented by the formulas (C) to (E) indicated below). In addition, although urea is generally added in excess to obtain N-(aliphatic)-substituted-O-alkyl urethane at high yield, since the residual excess urea itself undergoes a thermal decomposition reaction at temperatures of 130° C. or higher, isocyanic acid and ammonia gas are generated (see, for example formula (F) indicated below), or the isocyanic acid forms biurets that further undergo thermal decomposition at temperatures of 200° C. or higher (see, for example, formulas (G) and (H) indicated below), thereby contributing to the formation of polymers and the like (see, for example, formulas (I) to (L) indicated below). Since these polymers and the like have extremely low solubility in solvents and the like, they frequently adhere or solidify to the reaction vessel, thereby making these methods industrially unsatisfactory. In addition, since there is no description regarding recovery of the urea or carbonic acid derivative used in excess, increases in the amount of urea or carbonic acid derivative used were unable to be avoided.
(wherein R represents an organic residue, R′ represents an aliphatic group, and a represents an integer of 0 or more.)
Furthermore, for the sake of simplifying the explanation, although the above formulas indicate reactions in the case R represents a divalent organic residue and R′ represents a monovalent organic residue, it can be easily surmised that similar reactions proceed even in the case both R and R′ have a valence of 2 or more.
With respect to this point, N-substituted carbamic acid-O-aryl esters are known to easily decompose to their corresponding isocyanates and aromatic hydroxy compounds (see, for example, Non-Patent Document 4), and several methods for producing N-substituted carbamic acid-O-aryl esters have been disclosed.
Patent Document 7 discloses a method for producing an aliphatic N-substituted carbamic acid-O-aryl ester by a one-step reaction of urea, an aromatic hydroxy compound and an aliphatic primary amine. Patent Document 8 discloses a method for producing an N-substituted carbamic acid-O-aryl ester by reacting urea and an aromatic hydroxy compound in a first step followed by reacting with a primary amine in a second step.
In these methods as well, it is necessary to use an excess amount of urea or carbonic acid derivative with respect to the amino group of the aliphatic amine in order to improve yield based on the comparatively expensive aliphatic amine. However, since these patent documents also do not describe recovery of the urea or carbonic acid derivative used in excess, increases in the amounts of urea or carbonic acid derivative used were unable to be avoided.
Patent Document 9 discloses a method for producing an aliphatic N-substituted carbamic acid-O-aryl ester from an aliphatic primary polyamine, an aromatic hydroxy compound and urea and/or non-N-substituted carbamic acid-O-aryl compound, wherein the non-N-substituted carbamic acid-O-aryl compound is recovered from the resulting urethanation reaction solution and recycled as a raw material of the urethanation reaction. According to this method, an attempt is made to inhibit increases in basic units of the urea or non-N-substituted carbamic acid-O-aryl compound. This method comprises obtaining an aromatic hydroxy compound and isocyanic acid by thermal decomposition of a non-N-substituted carbamic acid-O-aryl compound contained in an urethanation reaction solution, reabsorbing the isocyanic acid formed by decomposition into the aromatic hydroxy compound, and then reacting with the aromatic hydroxy compound to recover the non-N-substituted carbamic acid-O-aryl compound. However, in addition to the procedure being complicated, the recovery rate of the non-N-substituted carbamic acid-O-aryl compound was unable to be made adequately satisfactory.
In the case of all of the methods described above, it is difficult to quantitatively obtain N-substituted carbamic acid-O-aryl ester by using urea and a non-N-substituted carbamic acid ester as raw materials, and not only are various structures of polymers formed (and in many cases containing polymers for which the structures thereof are unable to be identified), these polymers adhere to the reaction vessel or, as a result of these compounds being formed, there was the problem of increases in the amounts of urea and amine compounds used. In addition, when producing isocyanates by applying N-substituted carbamic acid-O-aryl ester to a thermal decomposition reaction, these polymers additionally form other polymers by reacting with isocyanates formed by thermal decomposition, which may also cause problems due to adhering to or solidifying in the reaction vessel.
Therefore, methods have been disclosed for enabling a solvent to be present during thermal decomposition of N-substituted carbamic acid-O-aryl esters or N-substituted carbamic acid-O-alkyl esters, for example, to avoid the problem of adhesion and solidification of polymers to the reaction vessel.
For example, according to the description of Patent Document 10, thermal decomposition of an aliphatic, alicyclic or aromatic polycarbamate is carried out at 150 to 350° C. and 0.001 to 20 bar in the presence of an inert solvent and in the presence or absence of a catalyst, auxiliary agent in the form of hydrogen chloride, organic acid chloride, alkylating agent or organic tin chloride. By-products formed can be removed continuously from the reaction vessel together with the reaction solution, for example, and a corresponding amount of fresh solvent or recovered solvent is added simultaneously. However, a disadvantage of this method is that, for example, a decrease in the production efficiency of polyisocyanate occurs due to the use of a refluxing solvent, and what is more, a large amount of energy is required, including that for recovering the solvent, for example. Moreover, the auxiliary agent used is volatile under the reaction conditions, thereby potentially contaminating the decomposition product. In addition, the amount of residue is large based on the amount of polyisocyanate formed, thereby making economic efficiency and reliability as an industrial method suspect.
Patent Document 11 describes one method for continuous thermal decomposition of a carbamate, such as an alicyclic diurethane in the form of 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane, supplied along the inner surface of a tubular reaction vessel in a liquid form in the presence of a high boiling point solvent. This method has the disadvantages of low yield and low selectivity during production of a (cyclic) aliphatic diisocyanate. In addition, there is no description of a continuous method accompanying recovery of rebonded or partially decomposed carbamate, and post-treatment of solvent containing by-products and catalyst is also not mentioned.
On the other hand, the description of Patent Document 12, for example, relates to a circulation method for producing (cyclic) aliphatic diisocyanate by converting a corresponding diamine to an N-substituted carbamic acid-O-alkyl ester followed by thermal decomposition of this N-substituted carbamic acid-O-alkyl ester as an example of a method for carrying out thermal decomposition of an N-substituted carbamic acid ester without using a solvent. This method minimizes the decrease in yield by recirculating the product from an N-substituted carbamic acid ester decomposition step to an N-substituted carbamic acid-O-alkyl ester formation step following reaction with alcohol. By-products that are unable to be recirculated are removed by distillative separation of a reaction mixture of the N-substituted carbamic acid-O-alkyl ester formation step. In this case, worthless residue forms in the form of bottom products while all comparatively low boiling point components, including N-substituted carbamic acid-O-alkyl ester, are removed from the top of the column. However, this method has the disadvantage of using a large amount of energy. This is because all of the N-substituted carbamic acid-O-alkyl ester is required to be evaporated in the presence of a catalyst, and this N-substituted carbamic acid-O-alkyl ester must also be evaporated at a temperature level within a range of the decomposition temperature of the N-substituted carbamic acid-O-alkyl ester. Isocyanate groups formed in useful products react with residual carbamic acid ester groups, frequently resulting in the formation of comparatively high molecular weight by-products that cause a reduction in yield, thereby continuing to fail to solve the problem of adhesion and solidification of polymers to the reaction vessel.
In addition, according to the description of Patent Document 13, a method is disclosed whereby worthless by-products are partially removed prior to carrying out thermal decomposition of N-substituted carbamic acid ester. The disadvantage of this method is that the yield of isocyanate decreases as a result of N-substituted carbamic acid ester being contained in the partially removed by-products. In addition, since polymeric compounds form and adhere to the reaction vessel as a result of heating of by-products remaining in the reaction vessel without being discharged from the reaction vessel, the problem of adhesion and solidification of polymers to the reaction vessel remains unsolved, and long-term, continuous operation is difficult.
As has been described above, a method for thermally decomposing a non-N-substituted carbamic acid-O-aryl ester contained in a reaction solution of an N-substituted carbamic acid ester production step to obtain an aromatic hydroxy compound and isocyanic acid, reabsorbing the isocyanic acid formed by decomposition in the aromatic hydroxy compound, and recovering a non-N-substituted carbamic acid-O-aryl compound by reacting with the aromatic hydroxy compound (see Patent Document 9) and a method for purifying by crystallization (see Patent Document 14) have been developed as described above to solve the problems. However, in the case of the former method, it is difficult to adequately reduce the amount of non-N-substituted carbamic acid-O-aryl compound in the reaction solution of the N-substituted carbamic acid ester production step. In addition, in the latter method employing crystallization as well, it is difficult to selectively crystallize compounds having a similar structure at high yield, while also resulting in the problem of consuming energy to separate a solid-solution and recover the crystallization solvent. In addition, a method has been disclosed for removing urea and carbonic acid derivatives from a reaction solution of an N-substituted carbamic acid ester production step more easily in which an amine compound, urea and alcohol are reacted, the resulting solution of N-substituted carbamic acid-O-alkyl ester is introduced into a distillation column, and urea and carbonic acid ester are recovered from the distillation column (see Patent Document 15). However, due to the low boiling point of the alcohol used, there are limitations on the set temperature and set pressure of the distillation column, thereby reducing the amount of urea in the N-substituted carbamic acid-O-alkyl ester solution, while the effect of inhibiting formation of by-products is not necessarily clear.
For example, it is described in Patent Document 16 that when an O-alkyl urethane obtained by reacting carbonic acid ester and organic amine is subjected to thermal decomposition in the presence of an aromatic hydroxy compound, a minute amount of carbonic acid derivatives are also present. Here, the effect of the carbonic acid derivatives is to improve thermal stability of the aromatic hydroxy compound, and is not intended to have an effect on an N-substituted carbamic acid-O-alkyl ester or an isocyanate formed during thermal decomposition. Moreover, there is no description regarding an effect on N-substituted carbamic acid-O-aryl ester. In addition, although Patent Document 15 describes a composition for transfer and storage of N-substituted carbamic acid-O-aryl ester that maintains the stability thereof by inhibiting a thermal denaturation reaction of N-substituted carbamic acid-O-alkyl ester, as well as an isocyanate production process that uses that composition, there is no mention of residual urea of urea-derived compounds as described above in that composition, and there is also no mention made of N-substituted carbamic acid-O-aryl ester.    Patent Document 1: U.S. Pat. No. 4,297,501    Patent Document 2: U.S. Pat. No. 3,873,553    Patent Document 3: U.S. Pat. No. 2,677,698    Patent Document 4: U.S. Pat. No. 4,713,476    Patent Document 5: European Patent Application No. 0568782    Patent Document 6: European Patent Application No. 0657420    Patent Document 7: U.S. Pat. No. 4,925,971    Patent Document 8: Japanese Patent Application Laid-open No. H4-164060    Patent Document 9: Japanese Patent Application Laid-open No. H7-157463    Patent Document 10: U.S. Pat. No. 4,388,246    Patent Document 11: U.S. Pat. No. 4,692,550    Patent Document 12: European Patent Application No. 0355443    Patent Document 13: Japanese Patent No. 3382289    Patent Document 14: Japanese Patent No. 2804232    Patent Document 15: WO 2008/120645    Patent Document 16: WO 2008/084824    Non-Patent Document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870    Non-Patent Document 2: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 649, 1870    Non-Patent Document 3: Journal of American Chemical Society, Vol. 81, p. 2138, 1959    Non-Patent Document 4: O. Bayer, Das Diisocyanat-Polyaditions Verfahren, p. 12, 1963    Non-Patent Document 5: Journal of Synthetic Organic Chemistry, Japan, Vol. 20, No. 11, p. 1003, 1962